Catalytic dehydrogenation process



Patented Oct. 16, 1945 Karl H. Hachmuth, Bartlesville, Okla, assignor to Phillips Petroleum Company, a. corporation of Delaware No Drawing. Application September 20, 1943, Serial No. 503,151

6 Claims.

genation product, butylenes, while the activity.

of the catalyst is high. Lowering the temperature of the catalyst bed as the catalyst becomes deactivated reduces losses from thermal cracking and coking.

In conventional dehydrogenation the temperature is maintained at a substantially constant level. This constant temperature method offers several difficulties, If the temperature is held constant at a high level, excessive side reactions such as thermal cracking and coking occur. The

higher the level at which temperature is maintained, the more pronounced is the occurrence of these undesirable reactions. For example, the

amount of thermal cracking will approximately double for every 50 F. increase in temperature. These undesirable side reactions result in'a, decrease in the amountof feed stock available for recycle, thus reducing the ultimate yield (emciency) of the process. In addition, these side reactions increase the hydrocarbon load which must be handled insubsequent separation steps. Another disadvantage of high temperature operation is that the coking apparently causes more rapid deactivation of the catalyst due to the increased deposition of carbon on the surface of the catalyst. If the temperature is held constant at a low level (low temperature for dehydrogenation of n-butane being anything below 1100 FL), temporary initial catalyst poisoning is evident. This poisoning is apparently due to moisture ad- .sorbed on the surface of the catalyst during the regeneration cycle. If dehydrogenation is carried out at high temperatures, this moisture is removed so rapidly that no harmful efiects are evident. However, at low temperatures removal of the poisoning is much slower and as a result the initial yield of the desired dehydrogenation product is lower than at subsequent points during the dehydrogenation cycle. Since'the yields should be greatest at the beginning of the dehydrogenation cycle when the activity of the catalyst tained from low temperature dehydrogenation are lower than those for the high temperature process wherein temporary initial catalyst poisoning is quickly removed.

It was found that by beginning the dehydrog'enation cycle at a high temperature and then. slowly lowering the temperature as the catalyst became deactivated, it was possible to obtain both,

high yields and high efficiencies. The results were exactly opposite to results which were expected on the basis of conventional methods and. fundamental chemical reactions. Since dehydroenation is 'an endothermic reaction, and endothermic reactions are favored by the addition of heat, it would appear that the temperature should be raised not lowered in order to maintain a satisfactory yield as the activity of the catalyst declined. However, this was not the case as shown by this improved dehydrogenation process. Thus this process constitutes something new and unantic pated by workers in the held of catalytic dehydrogenation.

The process of this invention is advantageous since high yieldscan be obtained without sacriflclng efliciency. Another advantage of this invention is the removal of temporary initial catalyst poisoning by means of the high temperatures used at the beginning of the dehydrogenation cycle.

In this process it is possible to obtain increased yields of the desired product while the activity of the catalyst. is high by operating at initial temperatures which are sufliciently high to shift the thermodynamic equilibrium and thus increase the extent of dehydrogenation. This is possible since the dehydrogenation reaction is endother- 'mic .in nature'and therefore is favored by the addition of heat. When the activity of the catalyst is high, thermodynamic equilibrium is ap-.- parently the factor which limits the extent of dehydrogenation. Thus at higher temperatures,

it is possible to effect increased dehydrogenation by a favorable shift of the thermodynamic equilibrium However, as the catalyst becomes deactivated, apparently other factors than the thermodynamic equilibrium tend to limit the extent of dehydrogenation and thus it is possible to dehydrogenate only that per cent of the nbutane feed stock determined by these limiting factors. For example, if some factor such as diffusion limits the dehydrogenation to 25 per cent, then no matter if the temperature is raised to a point corresponding to 50 per cent dehydroenation, the yield will still be only 25 per cent.

is at its peak. it is evident that the yields 010- Since the high temperature can no longer effeet increased yields by shifting the equilibrium but can effect'increased thermal cracking and coking, such high temperatures are no longer desirable.

One object of this invention is to ofier a means for catalytic dehydrogenation of saturated aliphatic hydrocarbons. vention is to disclose improvements in a process for the dehydrogenation of n-butane to produce butylenes. Still another object is to obtain as high yields of butylenes as are obtained in higher temperature dehydrogenation without sacrificing the elliciency of the process. Still another object of this invention is to eifect a, shift of thethermodynamic equilibrium by means of high initial temperatures and thus take advantage of the endothermic nature of the dehydrogenation reaction while catalyst activity is high, in order to obtain higher yields of the desired product, butylenes. Still another object of this invention is to remove temporary initial catalyst poisoning by means of the higher temperatures used at the beginning of the dehydrogenation cycle.

In accordance with the present invention, the catalytic dehydrogenation of parafiins to olefins is accomplished by contacting the paraffin with a dehydrogenation catalyst under conditions which effect dehydrogenation of the paraflin to thecorresponding olefin. The treatment is initiated .at a relatively high temperature and as the dehydrogenation proceeds the temperature is lowered. The treatment may be carried out by passing a stream of the parafiln preheated to a suitable temperature through a bed of solid dehydrogenation catalyst the temperature of which is controlled in any suitable manner. Alterna- Another object of this in- I tively a fluid catalyst may be used. The parailin is usually relatively low-boiling, i. e. having at least two but not more than six carbon atoms, these being ethane, propane, normal butane,.isohutane, normal pentane, neopentane, isopentane and the several hexanes. principal paraflin dehydrogenation stock'in the I present state of the art.

The temperature at the beginning is sufliciently high to overcome the temporary initial poisoning Normal butane is the at such a rate as to minimize losses of hydrocarbon feed stock by thermal cracking and coking.

In a preferred embodiment normal butane. is dehydrogenated over a catalyst composed of alumina impregnated with chromium and magnesium oxides with the temperature initially at yields indicates that the additional heat resulted 1160 F. and the temperature is reduced at the rate of 1 F. per minute for one hour. The dehydrogenation is then discontinued, the catalyst regenerated and the process repeated. The temperature referred to is the external tube wall temperature of 2 inch ID (inside diameter) tubes.

EXAMPLE sium oxides.

crease? cycle, during which dehydrogenation occurs, the temperature is approximately 1100" F.

The following table contains some data on the improved dehydrogenation process of this inven-' tion. For purpose of comparison, data for both low and high constant temperature dehydrogenation processes are included.

1 This is the average temperature of the external wall of the catalyst tube.

1 Average pressure in pounds per square inch absolute.

8 Space velocity in cubic feet, at standard conditions, of n-butanc per hour, per cubic foot of catalyst.

The dehydrogenation was carried out during one hour cycles using a catalyst composed of aluminaimpregnated with chromium and magne- The temperatures were measured on the external wall of the catalyst tube. However, actual internal temperatures'of dehydrogenation bear a direct relation to these external wall temperatures; The temperatures given apply to 2 inch ID tubes. Pressure, space velocity, and catalyst activitywere maintained at substantiallythe same levels. However, due to the difference in completeness of catalyst regeneration, it was not possible to return'the catalyst to identical activity in each case. 'A comparison of the initial yield of 27.2 per cent butylenes and the total per-pass yield of.27.43 per cent shows the eifect of initial catalyst poisoning on the yield in low temperature dehydrogenation.

A comparison of the eificiencies for the two high temperature cycles shows the efiect of increased temperature on the efliciency. At around 1150 F. the eiliciency was 78.75 per cent, while at approximately 1190 F. it was only 59.88 per cent. On the other hand, comparison of the in increased destruction of feed stock without giving increased yields of butylenes.

By comparison, it can be seen that the yield of butylenes, 34.01 per cent, in the improved process was higher than that of the conventional processes, namely 27.43 per cent, 31.95 per cent, and 33.95 per cent, while the efficiency was 80.43 per cent for the improved process as compared with 59.88 per cent and 78.95 per cent for the high temperature cycles and 83.35 per cent for the low temperature cycle.

Thus it is evident that by means of this improved process for catalytic dehydrogenation of saturated hydrocarbons, a good yield can be obtained while maintaining the efilciency of the process at a high level. In addition, loss of hydrocarbon feed stock through coking and thermal cracking is reduced; the eflect of temporary assess? improved process are not necessarily the best values. Choice of the optimum conditions will depend upon the nature oi. the catalyst used and upon characteristics of the dehydrogenation equipment itself. Such optimum conditions must be found by experiment.

While the temperatures given herein were measured on the external walls of 2 inch ID tubes; it will be understood that catalyst tubes of any desired diameter may-be employed in carrying,

out the process of the present invention. In the case of tubes of a diiierent size, slight variations from the temperatures given may be employed .as found desirable. It is also to be understood that the invention is not limited in its application to 3. A process" for catalytically dehydrogenating normal butane to butylenes which comprises passing a stream of normal butane through 'a bed or dehydrogenation catalyst and thereby dehy-t drogenating said normal butane to butylenes,

, initiating said treatment with said catalyst at the use of a catalyst case comprising tubular catalyst chambers but is applicable with appropriate modification to any form or catalytic dehydrogenation equipment. The selection of suitable types and sizes oi equipment, temperature and other operating conditions'will be obvious to those skilled in the art in the light of this disclosure.

I claim:

l. A process for catalytically dehydrogenating 2., iii to oleflns which comprises contacting the paramn with a dehydrogenation catalyst under conditions eflecting dehydrogenation of said paramn to the corresponding. olefin, initiatins said treatment at a high temperature, and as said dehydrogenation proceeds and said catalyst graduaily becomes deactivated gradually lowering said temperature from the beginning to the end oi the on-stream period.

2. A process for catalytically dehydrogenating mramns to oleflns which comprises contactin the paramn with a dehydrogenation catalyst under conditions eflecting dehydrogenation of said paramn to the corresponding olefin, initiating said treatment with said catalyst at a high temperature such as to overcome temporary initial poisoning oi said catalyst and to give high yields while the activity of the catalyst is high, and as said dehydrogenation proceeds and said catalyst gradually becomes deactivated gradually lowering said temperature from the beginning-to the end of the on-stream period at such a rate as to minimize losses of hydrocarbon feed stool; by thermal cracking and coking. 1

a high temperature, and as said dehydrogenation proceeds and said .catalyst gradually becomes deactivated progressively lowerin said temperature from the beginning to the end oi the onstreamperiod.

4. A process for catalytically dehydrogenating normal butane to butylenes which comprises passing'a streamoi normal butane through abed of dehydrogenation catalyst and thereby dehydrogenating said normal butane to butylenes, initiating said treatment with said catalyst at a high temperature such'as to overcome temporary initial poisoning oi said catalyst and to give hish yields of butylenes while the activity of the catalyst is hi h, and as said dehydrogenation proceeds and said catalyst becomes gradually deactivated gradually lowering said temperature from the begining to the end oi the on-stream period at such a rate as to minimize losses oi hydrocarbon iced stock by thermal cracking and coking.

5. A process for catalytically dehydrosenating normal butane to butylenes which comprises passil'lg a stream of normal butane through a bed of solid dehydrogenation catalyst and thereby eifecting catalytic-dehydrogenation of said normal butane to butylenes, carrying out said dehydrogenation with said catalyst initially at a tempera= tureof about 1160 F. and as soon as said dehydrogenation is initiated progressively lowering said temperature from the beginning to the end of the on-stream periodv at a rate of approximately 1 F. per minute, continuing in this manner for approximately one hour, then discontinalyst, and repeating said process.

' 6. A process for catalytically dehydrogenating normal butane to butylenes which comprises passing a stream of normalbutane through a bed oi solid dehydrogenation catalyst composed oi alumina impregnated with chromium and magnesium oxides and thereby enacting catalytic dehydrogenation of said normal butane to butyl enes, carrying out said dehydrogenation with said catalyst initially at a temperature of about 1160 F. and as soon as said dehydrogenation is 

